Technical Field
This invention relates to a position detecting device of a capacitive system that detects a position indicated by a position indicator by receiving a signal from the position indicator.
Description of the Related Art
Tablet information terminals equipped with touch panels are commonly used. For example, the electrostatic induction system disclosed in Patent Document 1 (Japanese Patent Laid-Open No. Hei 08-179871) is widely used. In Patent Document 1, plural electrode conductors are disposed vertically and horizontally on a substrate surface of a position detecting sensor. The intersections formed by these electrode conductors are sequentially selected and the signal intensity is obtained. Based on the signal distribution, a position indicated by, for example, a position indicator, such as a pen type position indicator (hereinafter, referred to as an indicating pen), is obtained.
In Patent Document 1, the indicating pen internally includes an oscillator circuit. An oscillation signal with a predetermined frequency from the oscillator circuit is applied to the plural electrode conductors of the position detecting sensor. Furthermore, in the position detecting device, the position indicated by the indicating pen on the input surface of the position detecting sensor is detected from the signal intensity (signal level) of an induced signal obtained from each of the electrode conductors disposed vertically and horizontally.
The above-described device is frequently used in combination with a display device, such as a liquid crystal display (LCD). In this case, noise generated by the display device gets mixed in with the signal such that the position of the indicator cannot be correctly obtained or an incorrect position is detected. This results in erroneous operation in some cases. As such, denoising is an important issue for a position detecting device of an electrostatic induction system.
A differential amplifier circuit is often used in related arts for denoising. Specifically, two electrode lines disposed along the same direction are simultaneously selected. One of the electrodes is connected to the positive-side input of the differential amplifier circuit and the other electrode is connected to the negative-side input of the differential amplifier circuit. Therefore, the noise component is canceled and only a signal difference generated by the indicator, such as an indicating pen, is detected. Examples of this method include techniques described in Patent Document 2 (Japanese Patent Laid-Open No. Hei 5-6153) and Patent Document 3 (Japanese Patent Laid-Open No. Hei 10-20992).
For a position detecting sensor of the related arts, the above-described plural electrode conductors are provided by forming conductor patterns as shown in FIG. 11 on a substrate 1 composed of, for example, resin.
Specifically, in FIG. 11, formed on the substrate 1 with, for example, a rectangular shape are plural first electrode conductors (hereinafter, referred to as Y-electrode conductors) Y1, Y2, . . . , Ym (m is an integer equal to or larger than two) extending along a horizontal direction (X-axis direction), and plural second electrode conductors (hereinafter, referred to as X-electrode conductors) X1, X2, . . . , Xn (n is an integer equal to or larger than two) extending along a direction intersecting the horizontal direction, specifically a vertical direction (Y-axis direction) orthogonal to the horizontal direction. In this case, an area in which the intersections of the plural Y-electrode conductors Y1 to Ym and the plural X-electrode conductors X1 to Xn are formed (area surrounded by a dotted line 2s in FIG. 11) serves as a detection area 2 for an indicator.
Furthermore, as shown in FIG. 11, the substrate 1 includes, as projecting parts, a Y-electrode terminal 3 and an X-electrode terminal 4 forming connection terminals with an external signal processing circuit. In the Y-electrode terminal 3, conductor patterns extended from each of the plural Y-electrode conductors Y1 to Ym are concentrated by being routed along the periphery of the substrate 1. Similarly, in the X-electrode terminal 4, conductor patterns extended from each of the plural X-electrode conductors X1 to Xn are concentrated by being routed along the periphery of the substrate 1.
As described above, in the related-art position detecting sensor, lead conductor patterns formed by being extended from the respective electrode conductors need to be routed around along the periphery of the substrate 1 to thereby concentrate the plural Y-electrode conductors Y1 to Ym and the plural X-electrode conductors X1 to Xn in the Y-electrode terminal 3 and the X-electrode terminal 4, respectively. Thus, an interconnect area 5 (indicated by the hatched lines in FIG. 11) for the lead conductor patterns (connecting lines), surrounded by the dotted line 5s, needs to be provided in FIG. 11.
Accordingly, the related arts have a problem that the substrate 1 is larger than the detection area 2 for the indicator because of the existence of the interconnect area 5. This results in lower convenience and design property.
In related arts, in the case of a position detecting device of an electromagnetic induction system in which X-direction and Y-direction loop coils are formed in a position detecting sensor and used, an attempt to eliminate the interconnect area 5 of the connecting lines is made by forming the connecting lines on the back surface side (side on which the loop coils are not formed) of the substrate by using through-holes. This is allowed because, in the case of the electromagnetic induction system, components that receive a signal from an indicating pen are the loop coils and it is difficult for the connecting lines to receive the signal from the indicating pen.
However, in the case of a position detecting sensor of a capacitive system, the connecting lines also receive the signal from the indicating pen. Therefore, there is a problem that it is difficult to detect the position of the indicator if the connecting lines are formed on the back surface side (side on which the X-electrode conductors and Y-electrode conductors are not formed) of the substrate by simply using through-holes.
For a position detecting sensor of a capacitive system, a differential amplifier circuit, which performs differential amplification of received signals from two electrode conductors on which similar noise is superimposed, is used to remove noise that is generated in the sensor from the received signals obtained from the electrode conductors. It is preferable that the distance between the two electrode conductors, each connected to a respective one of the non-inverting input terminal and the inverting input terminal of the differential amplifier circuit, are as short as possible. If there is a large distance between the two electrode conductors the similarity of noise is lost and the effect of the enhancement in the noise resistance obtained by the differential amplification cannot be achieved. The circuit scale also becomes large.